1. Field of the Invention
The present invention relates to a motor which can be applied to an optical scanning device or the like, used by a laser printer, a digital copying machine, a laser facsimile, and the like, and a method of manufacturing the motor, and particularly to a drive motor with a dynamic-pressure bearing which allows high-speed rotation of an optical member, and a method of manufacturing the drive motor.
2. Description of the Related Art
Generally, in an optical scanning device in which a light beam is scanned on a recording medium, an optical deflector is used in which an optical member such as a polygon mirror, a hologram disk, or the like (which will be hereinafter referred to as a polygon mirror) is provided to rotate at high speed by a drive motor, for example, a coreless motor in order that a light beam including information may be deflected and scanned in a predetermined direction.
In recent years, a drive motor used by an optical deflector requires high-speed rotation of 10,000 to 30,000 rpm or more due to high-speed operation and high image quality tendency of a laser beam printer or a digital copying machine. Therefore, as a bearing of a drive motor, a dynamic-pressure bearing is employed in place of a conventionally used ball bearing from the viewpoint of the duration of life of the bearing.
Conventionally, a motor used by the optical deflector as described above is constructed such that, as shown in FIGS. 37 through 43, a rotor 16 in which a polygon mirror is provided in a fixed shaft 14 formed upright on a base member 12 disposed on the side of a stator 10 is supported by a dynamic-pressure bearing, excitation-switching control is effected for a drive coil 20 serving as a coreless coil and formed on a coil substrate 18 disposed on the base member 12, and the rotor 16 is rotated due to magnetic force acting between the drive coil 20 and a main magnet 22 disposed on the side of the rotor 16.
As also shown in FIG. 38, the fixed shaft 14 is formed upright at the central portion of the base member 12 in the stator 10. Herringbone grooves 24 which form a dynamic-pressure bearing are formed on an outer peripheral surface of the fixed shaft 14.
The coil substrate 18 is disposed on the surface of the base member 12 where the fixed shaft 14 is formed. Six drive coils 20 are disposed at predetermined positions on the coil substrate 18 and a control circuit (not shown) for these drive coils 20 is also formed thereon.
Further, a yoke 28 is disposed to be accommodated within a shallow groove 30 formed in the base member 12 at a position on the coil base plate 18, corresponding to the drive coil 20, on the side opposite to that where the drive coil 20 is formed (i.e., at the lower side of the drive coil 20 as shown toward the front in FIG. 37). The yoke 28 is used to direct, toward the rotor 16, magnetic line of force generated by the drive coil 20 and turned to the base member 12.
As shown in FIGS. 37 through 39, a thrust magnet holder 32 is mounted onto the base member 12. The holder 32 is made of aluminum and has a rectangular configuration with a circular opening being provided at the center thereof. The magnet holder 32 is also disposed and positioned at a predetermined location on the base member 12 in such a manner that a fastening member 34 passes through each of through holes 36 respectively formed at four corner portions of the holder 32. A stepped portion having a cut of an L-shaped cross section is formed in a peripheral portion of the circular opening formed in the holder 32, and a stator-side thrust magnet 38 made of a nylon-resin magnetic material and formed in the shape of a ring having a rectangular cross-sectional configuration is stuck to the stepped portion by an adhesive agent.
The rotor 16 mounted to the stator 10 having the above-described structure is formed as shown in FIG. 37 and FIGS. 40 to 43. As shown in FIGS. 37 and 40, a rotating shaft 40 of the rotor 16 is formed in the shape of a hollow cylinder and is disposed around the fixed shaft 14 of the stator 10 so that the fixed shaft 14 passes through the rotating shaft 40. When the rotating shaft 40 rotates at high speed, a radial bearing which is a dynamic-pressure bearing is formed between the fixed shaft 14 and the rotating shaft 40.
A ring-shaped flange 42 made of aluminum is fixed by shrink fitting at a predetermined position on the outer periphery of the rotating shaft 40. The flange 42 is provided with a mirror mounting portion 44 and a polygon mirror 48 is fixed on a mounting surface 46 of the mirror mounting portion 44. The mounting surface 46 is formed to be perpendicular to a shaft core of the rotating shaft 40 at high accuracy. Further, the polygon mirror 48 is formed in the shape of a polygonal prism and a side surface portion thereof is formed as a mirror finished surface.
A driving main magnet 22 made of a nylon-resin magnetic material is stuck to the surface of the flange 42 corresponding to the drive coil 20 on the side of the stator 10 by an adhesive agent. As also shown in FIG. 43, the main magnet 22 is entirely formed in the shape of a ring and an opening peripheral portion 52 whose inner diameter is made larger in a stepped manner is formed in the opening portion of the magnet 22 on the side of the stator 10. Further, the main magnet 22 is divided into eight equal sections each at an angle of 45 degrees from the center and these sections are respectively polarized to have an N pole and an S pole so that adjacent sections have different poles.
As also shown in FIG. 40, an FG magnet 54 for generating a speed-of-rotation detecting pulse, having a small cylindrical shape and made of a nylon-resin magnetic material, is stuck by an adhesive agent to a portion of the rotating shaft 40 protruding from the flange 42 to the stator 10 in such a manner that one end surface the magnet 54 is attached to the surface of the flange 42. The FG magnet 54 is divided into eight equal sections each at an angle of 45 degrees from the center and these sections are respectively polarized to have an N pole and an S pole so that adjacent sections have different poles.
Further, a stepped portion 56 having an annular cut of a rectangular cross section is formed in an outer peripheral corner portion of the flange 42 on the side opposite to that of the stator 10. A ring-shaped rotor-side thrust magnet 58 made of a nylon-resin magnetic material is stuck by an adhesive agent to the stepped portion 56.
As shown in FIG. 37, the rotor-side thrust magnet 58 and the stator-side thrust magnet 38 are provided to be coaxial with each other and are disposed adjacently at a predetermined interval. The outer peripheral surface of the rotor-side thrust magnet 58 and the inner peripheral surface of the stator-side thrust magnet 38 have different poles so that attractive force is generated therebetween, and a thrust magnetic bearing is thereby formed. The thrust magnetic bearing operates to float the whole rotor 16 in such a manner that attractive force acting between these magnets 38, 58 surpasses load in a thrust direction (i.e., an axial direction) of the rotating shaft 40 of the rotor 16.
For this reason, the rotor 16 is supported and received by the thrust magnetic bearing in the thrust direction and is also supported and received by a dynamic-pressure bearing in a radial direction. As a result, the rotor 16 is controlled by a drive circuit of the coil substrate 18 so that an alternating voltage is applied to the six drive coils 20, and high-speed rotation of the rotor 16 is allowed with the rotor 16 floating in the air.
The above-described optical deflector, particularly a motor thereof has the structure in that, on the side of the rotor 16, the main magnet 22, the FG magnet 54, and the rotor-side thrust magnet 58, which are each made of a nylon-resin magnetic material whose thermal expansion coefficient is different from that of the flange 42, are stuck to the flange 42 made of aluminum. For this reason, when, at the time of using the optical deflector, the rotor 16 is rotated at high speed and generates heat, thermal stress is generated between the flange 42 and each of the magnets 22, 54, 58 as shown in Table 1 below.
TABLE 1 ______________________________________ Data with regard to adhering of magnets ______________________________________ resin magnetic material aluminum (nylon) linear expansion coefficient linear expansion coefficient 50 .times. 10.sup.6 23.1 .times. 10.sup.-6 ______________________________________
Thermal stress at a portion where the resin magnetic material is stuck to an aluminum member: 0.01 kg/m.sup.2 PA1 Centrifugal stress at the portion where the resin magnetic material is stuck to an aluminum member: 0.062 kg/m.sup.2 (16,000 rpm)
Namely, the linear expansion coefficient of the flange 42 made of aluminum is 23. 1.times.10.sup.-6 and the linear expansion coefficient of each of the magnets 22, 54, 58 made of nylon resin is 50.times.10.sup.-6. Due to the heat generated when the rotor 16 is rotated at the speed of rotation of 16,000 rpm, the thermal stress acting at a portion where the flange 42 is stuck to each of the main magnet 22, the FG magnet 54, and the rotor-side thrust magnet 58 is 0.01 kg/mm.sup.2.
At the same time, due to centrifugal force generated when the rotor 16 rotates at the speed of rotation of 16,000 rpm, centrifugal stress of 0.062 kg/mm.sup.2 acts at the portion where the flange 42 is stuck to each of the main magnet 22, the FG magnet 54, and the rotor-side thrust magnet 58. As a result, the total stress of 0.072 kg/mm.sup.2 acts at the portion where the flange 42 is stuck to each of the main magnet 22, the FG magnet 54, and the rotor-side thrust magnet 58. For this reason, when the rotor 16 is used for a long time, the adhered portions are broken and these magnets 22, 54, 58 are respectively separated from the flange 42, which forms a hindrance to rotation of the rotor 16.
Further, during assembling and manufacturing of the rotor 16, the operation of adhering each of the main magnet 22, the FG magnet 54, and the rotor-side thrust magnet 58 to the flange 42 by using an adhesive agent requires a large number of operating processes and much time, thereby resulting in an increase of manufacturing cost.
Moreover, the rotor 16 is constructed such that the main magnet 22, the FG magnet 54, and the rotor-side thrust magnet 58, which are each formed as a separate body, are respectively mounted to the flange 42, and therefore, the inertia of the whole rotor 16 increases and initial unbalanced weight also becomes large.
For this reason, the operation of correcting balance of the rotor 16 becomes indispensable until the completion of assembling of the rotor 16, and therefore, the number of processes of the assembling operation increases and the manufacturing cost becomes high.
Next, the stator 10 in the optical deflector, particularly, in a motor has a structure in which the stator-side thrust magnet 38 made of a nylon-resin magnetic material is stuck to the aluminum holder 32, the magnet 38 being made of a material whose thermal expansion coefficient is different from that of the holder 32. For this reason, when, due to heat generated during the rotation of the rotor 16, thermal stress of 0.01 kg/mm.sup.2 operates, as shown in Table 1 described above, at the portion where the holder 32 is stuck to the stator-side thrust magnet 38, the adhered portion is broken and the stator-side thrust magnet 38 is thereby separated from the holder 32. This may form a hindrance to the rotation of the rotor 16.
Further, the operation of adhering the stator-side thrust magnet 38 to the holder 32 requires a large number of operation processes and much time, and therefore, the manufacturing cost increases.
Moreover, in order that the interval between the stator-side thrust magnet 38 which forms the thrust bearing and the rotor-side thrust magnet 58 be correctly maintained, it is necessary that the holder 32 be positioned at high accuracy so as to be fixed to the base member 12 such that the stator-side thrust magnet 38 and the fixed shaft 14 of the base member 12 are made coaxial with each other, which requires a great deal of time.
In the whole optical deflector, particularly in the whole motor, the flange 42, the main magnet 22, the FG magnet 54, the rotor-side thrust magnet 58, the holder 32, and the stator-side thrust magnet 38, which are each formed as a separate body, are used as component parts of the optical deflector. For this reason, the number of parts increases and the manufacturing cost thereby becomes higher.
In order to solve the above-described problems, there may be considered each method disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 4-204625, 6-165460, and 6-123848 which have been conventionally proposed. The method disclosed in JP-A No. 4-204625 improves the balance of a rotor by integrally forming a flange, a main magnet, and an FG magnet so as to lessen the inertia of the rotor. The method disclosed in JP-A No. 6-165460 allows reduction in each number of parts and assembling processes by integrally forming the main magnet and the flange portion and does not require a balance correcting operation. Further, the method disclosed in JP-A No. 6-123848 allows reduction in each number of parts and assembling processes by integrally forming the main magnet and the flange portion from resin magnetic materials, thereby resulting in that the balance correcting operation is completed at one time.
However, the above-described methods each use a rolling bearing as the bearing of the rotor, and none of these methods has a structure having a thrust magnetic bearing inherent in the dynamic-pressure bearing. Accordingly, even when either of these methods is used, it is not possible to solve the problems in that the adhered portion of a stator-side or rotor-side thrust magnet inherent in the optical deflector having the dynamic-pressure bearing which also serves as the thrust magnet bearing is separated, or that the manufacturing cost increases due to a large number of assembling processes and difficult operation.
Further, the optical deflector in which the rolling bearing is used allows working speed of rotation up to 10,000 rpm or thereabouts. For this reason, even when the rotor has an integral structure made of a resin magnetic material, the deformation of the rotor due to the centrifugal force is small. However, in the optical deflector in which the dynamic-pressure bearing is used, the speed of rotation of 10,000 rpm to 30,000 rpm or more is allowed, and therefore, there is a possibility that, in a rotor having an integral structure made of only resin magnetic material, distortion occurs on a mirror surface of a polygon mirror due to the deformation of the rotor, which is caused by the centrifugal force. As a result, the structure of the optical deflector in which the rolling bearing is used cannot be simply applied.